1. Field of the Invention
The present invention relates to an apparatus for analyzing body fluids drawn from a living subject of the type having a fluid sample station connectable to the subject for drawing therefrom, a disposable cassette connectable to the fluid sample station for receiving a fluid sample to be analyzed, at least one sensor unit for analyzing the fluid sample and a control unit.
2. Description of the Prior Art
During intensive care, for example, it is important to collect information of the condition of the patient on a regular basis in order to provide an optimal treatment for the patient. Important information can be obtained by analyzing body fluids drawn from the patient, especially blood. A blood sample can be analyzed as to its blood gas content, e.g. partial pressure of oxygen (PaO.sub.2). Other parameters of interest in body fluids are pH, glucose, partial pressure of carbon dioxide (PaCO.sub.2), etc.
Historically, hospitals have made use of large central laboratories for analyzing samples taken from the patients, including blood samples. Although the laboratories can provide good measurements by using complex and expensive equipment, this system has some vital drawbacks when analysis of blood gas content is considered. The blood samples taken from the patient are not analyzed immediately and the result of the analysis can, therefore, not normally be reliably used for determining necessary and/or immediate changes in the treatment of the patient. The physician sometimes, particularly in acute cases, cannot await the result of an analysis of a sample which has to be transported to a laboratory.
In addition, such a conventional laboratory does not allow for the treatment Of patients with so called closed loop-systems (i.e. infusion, medication, ventilation, anaesthesia systems), for which the continuous or semi-continuous sample and analysis of body fluids is mandatory in order to acquire the necessary parameters for regulating the whole system.
Further, for parameters such as the blood gas content of blood, there is also the necessity of keeping the blood sample from contact with air which otherwise could affect the blood gas content in the blood sample.
As one approach to solving these problems, smaller mobile systems have been developed, i. e., point of care systems (for analyzing blood gas content). The point of care systems can be brought to the patient's bedside and usually include a measurement unit and a disposable cassette containing the sensors. A physician or nurse draws a blood sample from a patient into a cassette and analyzes it by means of the point of care system.
Although operating more flexibly and faster than the use of central laboratories, the point of care systems are ineffective in providing continuous or almost continuous information about the body fluid parameter, e.g the blood gas situation of the patient. In cases where treatment is given to the patient, using technical systems or equipment, the actual settings of the system or equipment could and should be made dependent upon the continuous information of the body fluid parameter, such as the blood gas content, and therefore require more regular samples to be taken and analyzed. "Continuous" body fluid measurements are a necessity if closed loop ventilator, infusion, anaesthesia, etc. treatment is to be given, e. g. automatic control of a lung ventilator based on measured blood gas content.
Systems for (practically) continuous measurement of blood gases are known. For instance U.S. Pat. No. 5,225,063 discloses an apparatus for measuring the partial pressure of oxygen, wherein a double lumen catheter is inserted into the patient's blood system. The outer lumen extends further into the blood system than the inner lumen, thereby almost enclosing the inner lumen. Blood can be drawn into the inner lumen, by a first pump, up to a measuring cell where a measurement is made. After measurement a second pump will pump a rinse medium through the outer lumen toward the blood system and the rinse medium will be pumped into the inner lumen to cleanse the lumen and the measuring cell from all blood residues before next blood sample is taken.
Another system of a similar kind is described in U.S. Pat. No. 4,535,786. In this system the blood sample is drawn to a measuring cell and is returned to the patient together with rinse medium after measurement.
These (almost) continuous measurement systems have some disadvantages. The sensors used in the measuring cell must be stable over a long time and usually require regular calibration as well. Such systems normally also require a very exact pumping system for drawing the blood sample, rinse medium and calibration solution in exact volumes. This requires exact (and expensive) pumps or advanced measurement systems for measuring the pumped volumes. Further, when frequent samples are taken, the successive minimum volumes required by the pump system additively result in an unacceptable loss of blood. Depending on the system design, some systems will lead to a loss of blood while rinse medium and calibration solution will enter the blood system. In addition, these systems also carry an inherent risk for the patient, since both the rinse medium and the calibration solution must always be completely harmless substances. In general, these systems have the inherent disadvantage of being "open" in the sense that these systems "communicate" body fluids from and to the body arid from and to the system and can therefore be a potential source of infection and/or necessitate the use of relatively complex and expensive elements, procedures (e.g. sterilization) and features in order to prevent these risks.